If it was a Kydex holster, it probably had retention. I don't own many kydex holsters, but all the ones I do own require a fair amount of force to remove them. I'd venture to say that the average person isn't going to be expecting it to be that secure, giving the OC'er enough time to react.
IMO, the serpa is absolutely NOT a good idea for OC. The release is positioned so that it's triggered by the index finger in a normal drawstroke. I think it was designed more to keep it from bouncing out than it was to protect against gun grabs.